Nowadays, in radio communications, and especially in mobile communications, various kinds of information such as images and data are transmitted as well as voice. Henceforth, demand for the transmission of various kinds of content is expected to continue to grow, further increasing the necessity of highly reliable, high-speed, large-volume transmission. However, when high-speed transmission is carried out in mobile communications, the effect of delayed waves due to multipath propagation cannot be ignored, and transmission characteristics degrade due to frequency selective fading.
Multicarrier modulation methods such as OFDM (Orthogonal Frequency Division Multiplexing) are attracting attention as one kind of technology for coping with frequency selective fading. A multicarrier modulation method is a technology that achieves high-speed transmission by transmitting data using a plurality of carriers (subcarriers) whose transmission speed is suppressed to a level at which frequency selective fading does not occur. With the OFDM method, the subcarriers on which data is placed are mutually orthogonal, making this the multicarrier modulation method offering the highest spectral efficiency. Moreover, the OFDM method can be implemented with a comparatively simple hardware configuration. For these reasons, OFDM is an object of particular attention.
Spread spectrum methods such as CDMA (Code Division Multiple Access) are another example of technology for coping with frequency selective fading. CDMA improves interference tolerance by spreading each user's information directly on the frequency axis with a user-specific spreading code to obtain spreading gain, and is already in use in mobile communications.
Recently, a method combining OFDM and CDMA (known as MC (multicarrier)-CDMA or OFDM-CDMA, but hereinafter referred to as “multicarrier CDMA”) has been attracting particular attention as an access method for implementing faster transmission. Multicarrier CDMA methods are broadly divided into a time domain spreading type, in which spread chips are placed on the time axis in each subcarrier, and a frequency domain spreading type in which spread chips are placed on the frequency axis at each time. In the former case, a path diversity effect is obtained but a frequency diversity effect is not obtained, whereas in the latter case, conversely, a frequency diversity effect is obtained but a path diversity effect is not obtained.
An example of a cell search method with this kind of multicarrier CDMA is described in “3-Step Cell Search Performance using frequency-multiplexed SCH for Broadband Multi-carrier CDMA Wireless Access” (Hanada, Atarashi, Higuchi, Sawahashi), TECHNICAL REPORT OF IEICE NS2001-90, RCS2001-91 (2001-07), pp. 73-78.
Here, in a broadband radio access method using multicarrier CDMA in the downlink, FFT (Fast Fourier Transform) window timing is first detected by means of a peak in the correlation values arising from a guard interval section and effective symbol section (first stage: FFT window timing detection). Next, FFT processing is performed using this FFT window timing, the correlation between the subcarrier component in which a Synchronization Channel (SCH) is multiplexed and an SCH replica is integrated over the length of one frame for each subcarrier, this correlation detected value is averaged by power addition in the frequency direction and time direction, and the timing at which the maximum correlation output after averaging is detected is detected as the frame timing (second stage: frame timing detection). Then, the correlation of each scrambling code is integrated in the time direction for each subcarrier using a Common Pilot Channel (CPICH) time-multiplexed at the detected frame timing, and a correlation value for each subcarrier is detected, in-phase addition is performed in the frequency direction, power addition is performed in the time direction, and a correlation value for each scrambling code is detected. The scrambling code for which the scrambling code correlation value is a maximum is detected, and the scrambling code is identified (third stage: scrambling code identification).
However, a problem with the above-described conventional cell search method is that, since the correlation value of the time-multiplexed common pilot channel and scrambling code is found for each scrambling code, the scrambling code at the time of this correlation value being a maximum is detected, and the scrambling code is identified, after frame synchronization is achieved, a cell search (achieving initial synchronization) takes time when there are a large number of scrambling codes. There is a further problem in that, even after initial synchronization, the same kind of processing as in the initial synchronization cell search must be carried out when another cell search is performed, which similarly takes time.